In recent years, in the production of semiconductor device or liquid crystal display device, the pattern formed therein has become rapidly finer owing to the progress of lithography technique. In order to achieve a finer pattern, it is generally being pushed forward to use an exposure illuminant of shorter wavelength. Specifically explaining, an ultraviolet ray typified by i line has been used; however, currently, a krypton fluoride (KrF) excimer laser (wavelength: 248 nm) is used mainly in the mass production of the device and an argon fluoride (ArF) excimer laser (wavelength: 193 nm) is being introduced in the mass production. Further, researches are under way on lithography techniques using, as a ray source (a radiation source), a fluorine dimer (F2) excimer laser (157 nm), an extreme ultraviolet ray (EUV), an electron beam (EB), etc.
In these lithography techniques, a chemically amplified positive type copolymer for lithography is used preferably. This copolymer has a repeating unit having a structure (hereinafter, this structure may be referred to as “acid-labile structure”) which has a polar group soluble in an alkali developing solution (hereinafter, this polar group may be referred to as “alkali-soluble group”), protected by a substituent group unstable in an acid and suppressing the dissolution in an alkali developing solution (hereinafter, this substituent group may be referred to as “acid-labile, dissolution-suppressing group”), and a repeating unit having a polar group for enhancing the adhesivity to semiconductor substrate or the like and adjusting the solubility in lithography solvent or alkali developing solution.
For example, in a lithography using a KrF excimer laser as the illuminant, there are known copolymers having a hydroxystyrene-derived repeating unit, a repeating unit having a hydroxystyrene-derived phenolic hydroxyl group protected by an acid-labile, dissolution-suppressing group (e.g. acetal structure or tertiary hydrocarbon group), or, a repeating unit having an (α-alkyl)acrylic acid-derived carboxyl group protected by an acid-labile, dissolution-suppressing group (e.g. acetal structure or tertiary hydrocarbon group), and other group (see Patent Literatures 1 to 4, etc.). There are also known copolymers having a repeating unit having an alicyclic hydrocarbon group as an acid-labile, dissolution-suppressing group in order to obtain improvements in dry etching resistance and the difference in the dissolution rates in alkali developing solution before and after exposure (see Patent Literatures 5 to 6, etc.).
In a lithography using an ArF excimer laser of short wavelength or the like as an exposure illuminant, there have been studied copolymers having no hydroxystyrene-derived repeating unit having a high absorption constant to a wavelength of 193 nm, and there are known copolymers having a repeating unit of lactone structure, as a polar group for enhancing the adhesivity to semiconductor substrate or the like and adjusting the solubility in lithography solvent or alkali developing solution (see Patent Literatures 7 to 10, etc.).
Meanwhile, in recent years, a technique of immersion lithography has been proposed. It is a technique in which exposure is conducted by filling a liquid (e.g. water) having a higher refractive index than air, between an object glass and a thin lithography film. As compared with the conventional lithography (hereinafter, this may be referred to as “dry lithography”) in which an air layer is present between an object glass and a thin film, the technique can enlarge the numerical aperture of lens for the same wavelength of ray source and can deepen the depth of focus for the same numerical aperture, and accordingly can form a finer pattern with the same wavelength of ray source. Therefore, immersion lithography using an ArF excimer laser as a ray source is being studied actively for practical use, and, as the copolymer used in the ArF immersion lithography, there is proposed the same copolymer as known in conventional ArF dry lithography (see Patent Literature 11 to 13, etc.).
In such a chemically amplified positive type copolymer for lithography, it is known that, when the acid value of a monomer (a raw material of polymerization) and a copolymer obtained by polymerization of the monomer is controlled at 200 mg-KOH/g (about 3.6 mmol/g) or less, the copolymer shows no change during storage and can form an intended pattern at a high accuracy (see Patent Literature 14). However, the lowest case of acid value reduction disclosed in the Patent Literature is 10 mg/KOH/g (about 0.2 mmol/g), and it has been unknown at all that, by reducing the acid value of copolymer to a level lower by at least one digit, there can be obtained a composition for semiconductor lithography superior in contrast of development rate to exposure energy (this contrast may hereinafter be referred to as development contrast and is a yardstick indicating the sharpness of resolution pattern).
Patent Literature 1: JP-A-1984-045439
Patent Literature 2: JP-A-1993-113667
Patent Literature 3: JP-A-1998-026828
Patent Literature 4: JP-A-1987-115440
Patent Literature 5: JP-A-1997-073173
Patent Literature 6: JP-A-1998-161313
Patent Literature 7: JP-A-1997-090637
Patent Literature 8: JP-A-1998-207069
Patent Literature 9: JP-A-2000-026446
Patent Literature 10: JP-A-2001-242627
Patent Literature 11: JP-A-2005-227332
Patent Literature 12: JP-A-2005-234015
Patent Literature 13: JP-A-2005-316259
Patent Literature 14: JP-A-2001-166481